Since the advent of clear moldable thermoplastics, it has been desired to prepare them in colored form, whether transparent, translucent, or opaque. Many processes exist in the art which have been evaluated to accomplish the coloration uniformly, efficiently, and cheaply. Among these exist processes for adding colorants in dry form to the molten thermoplastic, for adding colorants in dry form to a solid polymer and mixing in the molten state, and for adding colorants in solution or dispersion to a solid polymer as it is fed to an extruder for first melting and then mixing in the molten state.
It has been considered potentially cost-effective and energy-conserving if the colorant could be added to the molten matrix thermoplastic polymer as it is processed upon leaving the reactor wherein it has been prepared. Further, the method would offer much easier cleanup and color changes than when the colorant is added at the rear of the extruder, such as with the feed of pellets to be re-melted. Further, the matrix polymer would receive less thermal history and thus avoid potential degradation problems from excess heating prior to forming the final object. Further, such a process should allow better control and less variability of the colorant content, especially when low levels of colorant, such as for toning or optical brightening, are involved.
Such preparative/extrusive manufacturing processes are common for methacrylic polymers, that is, polymers with at least 80 weight percent of their units derived from methyl methacrylate, especially where the polymer is prepared by a continuous technology, unreacted monomer removed by devolatilization and recycled, and the devolatilized polymer conveyed directly to an extrusion process for forming pellets, film or sheet. Such a preparative/extrusive process is also common for polyglutarimides, wherein the molten methyl methacrylate polymer is reacted in an extruder with a primary amine, such as monomethylamine, to form the glutarimide units, optionally treated with an alkylating agent such as triethyl orthoformate or dimethyl carbonate to lower the acid and anhydride content, and then conveyed directly to an extrusion process for forming pellets, film or sheet.
However, a colorant process where the colorant is added to the molten polymer just prior to pelletization or extrudation or molding would offer similar advantages over other colorant methods, even if the polymer has been formed separately and cooled, then re-melted for coloration. For example, clear polymer of methyl methacrylate could be separately prepared in an appropriate molecular weight range and then re-melted, colored by direct colorant addition after melting, and then processed into colored melt-calendered sheet.
The difficulty with such a direct colorant-addition approach for methacrylic and glutarimide polymers is that there have been no known effective combination of colorants and non-volatile carriers known which meet the requirements for these plastics, viz., (a) compatibility with matrix polymer, as judged by visual appearance and good dispersion; (b) thermal stability in the melt, as judged by no loss of molecular weight of the matrix polymer and no discoloration of the matrix polymer; (c) no adverse effect on weathering behavior, as judged by retention of physical and appearance properties after accelerated or conventional outdoor weathering test, and (d) little or no loss in physical properties of the matrix polymer, such as heat distortion temperature.
European Unexamined Patent Application 478,987 teaches feeding of a liquid color concentrate to a polymer melt stream, mixing, and then devolatilizing to remove the carrier material. This application teaches utility for polyamides and polyesters only, not the polymers of the present invention. It further requires a devolatilization step, which is not a critical step of the present process. Indeed, the carrier in the '987 process is generally water, whereas the present invention utilizes carriers of sufficiently high boiling point that under the conditions employed for mixing and extruding, the carrier is not volatilized. Eliminating the need for devolatilization offers faster extrusion rates and less opportunity for remaining volatiles to cause poor moldings. Thus, the process described in the '987 application does not present a satisfactory solution to the problem of coloring methacrylic or glutarimide polymers in the melt directly after polymerization and/or reaction, and so avoiding the less satisfactory processes of feeding colorant prior to the polymerization or reaction, or re-melting the polymer after adding colorant to isolated pellets.
European Patent Specification 234,889 teaches that previous attempts to disperse colorant dispersions in carriers and then convey to a polymer melt have given unsatisfactory mixing, and teaches a solution to the problem wherein the colorant/carrier combination is admixed after the extrusion of the matrix melt is completed, utilizing a positive displacement feed means to form two streams of the melt and adding the colorant/dispersant into one stream. Their solution requires sophisticated equipment added externally to the end of the extruder.
We have now found that suitable non-volatile colorant carriers exist which, in combination with appropriate feeding and melt-mixing processes, will allow a wide number of colorants to be added and dispersed effectively in the "additives zone" of an extruder, with subsequent advantages in cost and ease of manufacture of colored plastics of high quality.